comorbid substance or alcohol abuse (Davis et al., 2005; Melartin et al., 2002). In their analysis of the origins of the comorbidity of substance use disorders, Swendsen and Merikangas (2000) considered whether they share a causal relationship (e.g., alcoholism causes depression or the reverse) or are related because of a shared etiological factor. Their data and review suggest a causal association, rather than shared etiology, for alcohol and depression, with evidence both for depression causing alcohol abuse and abuse causing depression. However, for other substance abuse, the patterns were inconsistent, suggesting that multiple mechanisms may be contributing to the comorbidity.

According to the Diagnostic and Statistical Manual of Mental Disorders, not only are Axis I disorders (i.e., clinical disorders, including major mental disorders, as well as developmental and learning disorders) highly likely to co-occur with depression, but also personality disorders are more the rule than the exception with depressed patients. Personality disorders refer to a set of patterns of dysfunctional conduct and attitudes that start early in life, are persistent, and affect all areas of a person’s functioning. Depending on the study, rates of personality disorders among depressed people range between 23 and 87 percent (Shea et al., 1990; Shea, Widiger, and Klein, 1992). Most studies have found that personality disorders in the “dramatic/erratic” cluster (such as borderline personality disorder) and in the “anxious/fearful” cluster (such as avoidant personality disorder) predominate (e.g., Alpert et al., 1997; Brieger, Ehrt, and Marneros, 2003; Rossi et al., 2001; Shea et al., 1990).

One of the crucial problems with depression co-occurrence with other disorders is that the combinations may greatly complicate both the clinical course of depression and the efficacy of typical treatments. For example, the presence of a comorbid anxiety disorder predicts a significantly worse course of depression and dysthymia (Brown et al., 1996; Gaynes et al., 1999; Shankman and Klein, 2002). Likewise, a comorbid personality disorder predicts a poorer outcome (Daley et al., 1999; Klein, 2003; Klein and Shih, 1998; see the review by Newton-Howes, Tyrer, and Johnson, 2006).

Depression is also a ubiquitous presence in medical illnesses, and a recent large depression treatment study (Sequence Treatment Alternatives to Relieve Depression: STAR*D) found that 53 percent of depressed patients had significant medical comorbidity (Yates et al., 2004). Serious acute and chronic diseases are highly stressful, and depression may be a reaction to the challenges associated with such problems; it can even result from the pathophysiological processes of certain diseases.

Of particular note is the role that depression may play as a contributor to ill health (Katon, 2003). For example, depression may interfere with healthy lifestyle choices, such as regular exercise, smoking cessation, good nutrition, and compliance with medical treatments; dysfunctional self-care